Large synchronous AC generators produce the AC power for many power grids throughout the world. These generators comprise a generator shaft, a rotor having a main field winding mounted on the shaft, a stator, and a frame for supporting the stator and the shaft. The rotor and stator each comprise iron conductors for magnetic flux. The stator has a stationary armature winding mounted on the inner surface of the stator frame. The stator armature winding produces the AC power from a rotating magnetic field provided by the rotor""s main field winding. DC current flowing in the main field winding produces this magnetic field. Mechanical torque applied to the generator shaft rotates the shaft and the main field winding. The main field winding""s rotating magnetic field induces the output AC current within the armature winding. This induced AC current is then applied to the power grid to increment the power provided by other similar generators.
Originally, slip rings were used to provide the DC current to the main field from a source external to the generator. While this design functioned adequately, the slip rings did require frequent maintenance. Newer designs avoid slip rings by providing DC power from an exciter generator mounted on the generator shaft and electrically connected to the main field winding.
The exciter generator usually contains a three-phase armature winding and rectifier bridge mounted on the rotor. A stationary field exciter winding surrounding the exciter armature receives from an external source, a DC current sufficient to produce a strong stationary magnetic field surrounding the exciter winding and the exciter armature. As the generator shaft rotates, AC voltage is induced in the exciter winding. Diodes connected in a bridge circuit rectifier convert the AC current induced in the exciter winding (exciter current) to DC. The DC exciter current output from the rectifier is then conducted to the main field winding by heavy buses.
Typically, the rectifier diodes are mounted on a special cylindrical carrier called a diode wheel. Typical diode wheels have a hub for attachment to shaft or the exciter armature and six radially projecting arms or spokes projecting from the hub. The spokes support a drum. The drum is typically eight to 12 in. wide and 15 to 24 in. in diameter depending on the current to be generated. Diode wheels are usually made from steel. The diodes are mounted on the interior drum surface and are electrically connected to each other to form the bridge circuit.
The diode wheel is mounted axially adjacent to the exciter armature and rotates with the generator shaft, serving in effect as a rotating carrier for the diodes. At least six diodes are typically mounted on the diode wheel in a 3-phase, full wave bridge rectifier circuit receiving AC from the exciter windings. The exciter current generated by the exciter generator and applied to the main field windings is large, in the range of hundreds or even thousands of amperes. Because of these large currents, the diodes and the large buses electrically connecting the diodes to each other and to the main field winding on the rotor must be able to carry substantial current and must operate reliably at high temperatures and under high centripetal forces. Minimizing diode temperature both increases the current capacity of the diodes and lengthens their life as well. To deal with this heat, it is important to provide a superior heat sink for these diodes.
In spite of suitable ambient temperature, the diodes occasionally fail and must be replaced. Power diodes of the type used in these exciter bridge circuits usually fail in a reverse conducting mode, which results in a short across the diode and a short across a phase of the exciter winding. This creates the potential of significant damage to the exciter winding. Because of this failure mode, the practice is to use two diodes in series as a single diode element in the bridge circuit so that if one diode fails the other of the pair will continue to function to protect the winding and allow normal operation. Once a diode fails however, the exciter is at risk should the second of the pair fail, so it is useful to continuously monitor the status of each diode.
However, monitoring diode status has problems. The diode wheel rotates in the enclosed space of a large power generator, and so the diodes it carries are not easily accessible. Slip rings can be used to conduct signals indicating diode status, but these too tend to wear and require regular preventive maintenance. In one system now deployed, status of the diodes and of other operating parameters as well for the main and exciter armatures are transmitted by RF signals to an adjacent receiver, avoiding the problem of slip rings.
As mentioned, a diode should be replaced relatively quickly if it fails. Of course, this requires shutting down the entire generator. Idling a generator for such maintenance or for any maintenance for that matter, takes an expensive generator out of service for a period of time and makes the installation less profitable. To minimize down time, each diode pair can be mounted in a module. Rather than having to loosen the mount for an individual diode in cramped quarters, the repairer simply unbolts the electrical and mechanical connections for the module from the inside of the drum and the then reverses the procedure with a new module.
The modules must mount securely to the drum interior because of the high centripetal forces developed by the high-speed rotation. For two reasons, the drum interior should be smooth and have internal geometry that precisely matches that of the contacting module surface. In the first place the matched geometry results in more direct metal-to-metal contact, improving heat transfer from the modules to the drum.
Secondly, when the module is bolted to the drum interior, the corresponding outer surface of the diode module is forced to conform to the drum surface. If the two surfaces do not have substantially identical geometry, the forces applied by the bolts flexes the module housing slightly, which may result in damage in the diode module. The only reasonable way we know to assure identical geometry is for the two surfaces to have substantially identical cylindrical radii of curvature and near-specular finishes.
However, we have found that it is difficult to machine the interior surface of the drum where the drum is integral with the spokes. A solution to this problem has been to construct the hub and spokes as one piece of the diode wheel and the drum as another piece, allowing the drum""s interior to be machined without interference from the spokes. The drum""s interior diameter is dimensioned to be very slightly smaller than the circle defined by the ends of the spokes when both are at room temperature. The drum is then heated sufficiently to expand it to a diameter slightly greater than the spoke ends. The drum is slipped onto the spokes, after which the drum is cooled causing it to shrink and fit tightly onto the hub and spokes.
While this design dramatically improves the fit of the drum surface to the modules, we find that shrink-fitting the drum to the spokes creates a new set of problems. Points of high stress in the spokes and the drum occur creating the potential for mechanical failure. The drum can distort from a perfect circle shape, causing balance and run-out problems during the high-speed rotation. These issues must be addressed to form a satisfactory two-piece diode wheel, and have created significant problems for us.
We have devised a two-piece diode wheel for use in a brushless exciter that solves many of these problems. As previously explained, such a diode wheel has a peripheral drum for carrying a plurality of diodes and a hub having an axis of rotation. The hub has a plurality of radially extending spokes contacting the interior of the drum and supporting the drum in a substantially fixed position relative to the hub. To reduce stress concentrations, we provide at the outer end of at least a first of the spokes, an outer surface having an area substantially greater than the minimum cross sectional area of the spoke. This substantially greater area of the outer spoke surface reduces stress concentrations in both the drum and the spoke.
In one preferred version of the invention, the drum has at room temperature a circularly cylindrical interior surface having a predetermined radius. Each spoke""s outer surface has at room temperature a radius of curvature relative to the hub""s axis of rotation, very slightly larger than the predetermined drum""s interior surface radius, and dimensioned to allow a shrink fit between the spoke ends collectively and the drum interior. The spokes"" outer surfaces collectively define a cylinder forming at room temperature an interference fit with the drum""s interior surface radius. In a preferred embodiment of this version, the at least first spoke has at its outer end a pad, said pad overhanging the first spoke at least angularly.